Chain-extendable crosslinkable urethane modified polyhydroxy oligomers and coating composition comprising same

ABSTRACT

High solids, solvent-based resin composition comprises novel chain-extendable, crosslinkable urethane modified polyhydroxy oligomers, crosslinking agent and, preferably, catalyst(s). The composition cures at elevated temperature to provide a coating on a substrate, such as steel, which is highly resistant to corrosion, humidity and solvents and provides corrosion protection for the substrate. The novel oligomers can be the reaction product of a polyol with a half-blocked diisocyanate, wherein said polyol comprises three or more hydroxy, and the half-blocked diisocyanate comprises the reaction product of an organic diisocyanate with approximately one molar equivalent of a monofunctional blocking agent.

RELATED CASES

This application is a continuation of application Ser. No. 334,798,filed Dec. 28, 1981, now abandoned.

This application is related to concurrently filed application Ser. Nos.334,792; 334,/93, now U.S. Pat. Nos. 4,409,380; 334,794, now U.S. Pat.Nos. 4,409,381; 334,795, now U.S. Pat. Nos. 4,403,086; 334,796 now U.S.Pat. Nos. 4,410,678; 334,797, now U.S. Pat. Nos. 4,410,679 and 334,842,now U.S. Pat. No. 4,396,756.

This invention relates to novel chain-extendable, crosslinkable urethanemodified polyhydroxy oligomers and to high solids, solvent-based,thermosetting coating compositions comprising same, which compositionsare useful to make coatings which are highly resistant to corrosion,humidity and solvents.

BACKGROUND OF THE INVENTION

Solvent based coating compositions are known which employ high molecularweight (e.g. 2,000 to 10,000) polymer resins having crosslinkingfunctionality, and a suitable crosslinking agent. Typically, suchcoating compositions are applied to a substrate, for example, byspraying, and are then cured by baking the coated substrate at anelevated temperature suitable to drive off the organic solvent and topromote the crosslinking reaction. The resulting thermoset coating, ifsufficiently humdity and solvent resistant, can provide aesthetic andfunctional advantages including corrosion protection for the underlyingsubstrate.

Coating compositions comprising such high molecular weight polymerresins typically comprise only 25% to 50% solids so as to be sprayableor otherwise conveniently applicable to a substrate. The viscosity ofcoating compositions of higher solids content is typically too high forthis purpose. Conventional epoxy ester based automotive vehicle sprayprimers, for example, typically have a volatile organic content ("VOC")of approximately 540 g/l.

Elimination of the volatile organic solvent portion during curing ofthese conventional low-solids coating compositions presents toxicity andin some cases flammability hazards. Furthermore, bulk volume of thesecoating compositions is relatively large and therefore presentsundesirable material handling difficulties, and added expense.Furthermore, excessive solvent losses and/or solvent recovery equipmentadd considerable expense to the coating operation. Recently,governmental regulations on hydrocarbon emissions, particularlyapplicable to automotive coating operations, mandate a significantreduction in volatile organic content for coating compositions. Thus,for example, governmental guidelines for 1982 presently require thatemissions of volatile organics from automotive vehicle primer coatingoperations be reduced to that equivalent to using coating compositionsof no greater than 350 g/l (2.9 lb./gal.) VOC. To meet that guideline,coating compositions of VOC greater than 350 g/l can be employed inconjunction with emissions treatment equipment to achieve the specifiedemissions limit. Such treatment presents significant additional expense,however, and thus there is a great need to provide coating compositionsof VOC reduced near to, or preferably even lower than, the 350 g/lgovernmental limit.

In response to these concerns, high solids coating compositions havebeen suggested which, typically, employ low molecular weightmulti-functional adducts or copolymers in combination withmulti-functional crosslinking agents. These high solids coatingcompositions are less viscous and, therefore, can be applied byspraying, for example, with far lower VOC than was possible withconventional epoxy ester based coating compositions or otherconventional coating compositions comprising high molecular weightpolymer resins. After application to the substrate, high solids coatingcompositions are cured by baking at a cure temperature, that is, at anelevated temperature suitable to drive off the volatile organic contentand to promote polymerization and crosslinking of the multi-functionallow molecular weight component(s).

Typically, high solids coating compositions yield cured coatings havingpolymeric networks that differ significantly in structure and morphologyfrom the polymeric networks provided by conventional, low solids coatingcompositions comprising high molecular weight polymers. Consequently,the physical properties of the coatings provided by such high solidscoating compositions can differ significantly from those of the curedcoatings provided by the conventional, low solids coating compositions.In particular, the cured coatings obtained from known high solidscoating compositions can be inferior in that they can be less flexible,less solvent resistant, less adherent to the substrate and/or for otherreasons provide less corrosion inhibition for the underlying substrates.Accordingly, it would be highly desirable to provide a coatingcomposition comprising low molecular weight materials suitable for usein high solids, solvent based coating compositions and yet which, uponcuring, form coatings having polymeric networks similar in structure andmorphology to those obtained with conventional low solids solvent-basedcoating compositions, and thus having physical properties comparable tothose obtained from conventional low solids solvent based coatingcompositions.

Accordingly, it is an object of the present invention to provide novellow molecular weight oligomers suitable for use in high solids,solvent-based coating compositions. In this regard, it is a particularobject of the invention to provide novel low molecular weight oligomerswhich are chain-extendable and crosslinkable during cure, in situ, onthe surface of a substrate to form polymeric networks similar instructure and morphology to those obtainable through use of conventionallow solids, solvent-based coating compositions.

It is another object of the invention to provide a novel coatingcomposition comprising such novel chain-extendable, crosslinkableoligomers. In this regard, it is a particular object of the invention toprovide a coating composition of sufficiently low VOC to meetgovernmental guidelines. It is also an object of the invention toprovide a coating composition which can be applied to a substrate byspraying or other known method.

It is another object of the invention to provide a method of making acoating on a substrate, which coating has a polymeric network similar instructure and morphology to that provided by conventional low solidssolvent-based coating compositions and having similar advantageousphysical properties including, for example, humidity and solventresistance and corrosion protection for the underlying subtrate.Additional aspects and advantages of the invention will be apparent fromthe following description thereof.

SUMMARY OF THE INVENTION

According to the present invention, low molecular weightchain-extendable, crosslinkable oligomers are provided which aresuitable for use in high solids, organic solvent based coatingcompositions. As used herein, a high solids coating composition is onecomprising polymerizable oligomers in which a volatile organic solventcontent of about 400 g/l (3.4 lb./gal.) or less yields a viscosity ofless than approximately 35 sec. #4 Ford Cup at 27° C. (80° F.).

The novel chain-extendable, crosslinkable oligomers of the invention areurethane modified polyhydroxy oligomers of number average molecularweight (M_(n)) between about 200 and about 1500, more preferably betweenabout 300 and about 1100. The oligomers comprise latent interreactivebifunctionality suitable for substantially linear chain-extensionpolymerization, in situ, on the surface of the substrate during cure ofthe coating. The oligomers further comprise hydroxy crosslinkingfunctionality. That is, the novel oligomers of the invention formcoatings on a substrate employing two distinct, independent reactions, achain-extension polymerization reaction, to form high molecular weighthydroxy functional polyurethanes and a crosslinking reaction betweensaid hydroxy functional polyurethanes (and/or the urethane modifiedpolyhydroxy oligomers before polymerization thereof) and a suitablecrosslinking agent.

More specifically, the urethane modified polyhydroxy oligomers comprisetwo or more hydroxy and a single blocked isocyanate functionality. Theoligomers preferably comprise the reaction product of a polyol with ahalf-blocked diisocyanate, wherein the polyol comprises three or morehydroxy groups and the half-blocked diisocyanate comprises the reactionproduct of an organic diisocyanate with approximately one molarequivalent of a monofunctional blocking agent.

According to the coating composition aspect of the invention, a highsolids, organic solvent based thermosetting resin composition comprisesthe novel chain-extendable, crosslinkable oligomers of the invention,suitable crosslinking agent such as, for example, aminoplastcrosslinking agent, and, preferably, a catalyst for the crosslinkingreaction and/or for the chain-extension reaction, and suitable organicsolvent such as, for example, butanol or other lower alkanol.

According to another aspect of the invention, a method of making acorrosion, solvent and humidity resistant coating on a substratecomprises applying to the substrate the high-solids, solvent-basedthermosetting resin composition of the invention comprising the novelchain-extendable, crosslinkable oligomers of the invention, and heatingthe resin compositions to between about 100° C. and about 190° C. andpreferably to between about 130° C. and about 150° C. for a periodsufficient to yield a cure coating.

DETAILED DESCRIPTION OF THE INVENTION

Preferred chain-extendable, crosslinkable urethane modified polyhydroxyoligomers of the invention are prepared by reaction of a polyol with ahalf-blocked diisocyanate. The polyol bears three or more hydroxylgroups, preferably about from 3 to 10 hydroxyl, such that the oligomerproduct of its reaction with the half-blocked diisocyanate provides anunreacted hydroxyl for chain-extension reaction as well as at least oneadditional unreacted hydroxy functionality for crosslinking reactionwith a suitable crosslinking agent such as, for example, an aminoplastcrosslinking agent. The polyol preferably has a molecular weight betweenabout 100 and 1000, and more preferably between about 300 and 700 toprovide an oligomer product which in solution provides suitably lowviscosity for use in high solids coating compositions. Exemplary polyolssuitable for the present invention include polyhydroxy functionalstraight or branched chain saturated or unsaturated hydrocarbons,optionally bearing one or more oxy or ester moieties and optionallybearing one or more heterocyclic atoms, aromatic and/or heterocyclicring, the heterocyclic atom(s) being selected preferably from N, O andS. Suitable polyol reactants include many commercially availablematerials well known to the skilled of the art.

Preferred chain-extendable, crosslinkable oligomers of the invention arethe reaction product of half-blocked diisocyanates with certainpreferred polyols. These preferred polyols comprise an epoxy-dioladduct, more specifically the reaction product of a suitable diepoxidewith diol. Preferred diepoxides are terminal diepoxides, that is,diepoxides bearing two terminal epoxide functionality, since these aregenerally more reactive and therefore require reaction conditions underwhich undesirable side reactions, for example, epoxy-epoxy reactions andgellation, can be more easily avoided. Most preferred in view of theircommercial availability are Bisphenol A epichlorohydrin epoxy resins,for example, Epon 828 (trademark), Shell Chemical Co., Houston, Tex.Other, higher molecular weight members of the Epon (trademark) seriesare suitable to make higher molecular weight oligomers of the inventionwhich provide coating compositions of somewhat higher viscosity (orlower solids content). It should be recognized, however, that the highermolecular weight members of the Epon series, for example Epon 1001 andEpon 1004, may be somewhat less preferred for preparing oligomers of theinvention, since the hydroxyl group(s) thereof can be less stericallyhindered and therefore more reactive. This can result in undersirableside reactions, for example, reaction between the epoxy functionality ofone diepoxide molecular and such hydroxy functionality of anotherdiepoxide molecule (rather than with an hydroxyl group of a diol). Theresult can be undesirable oligomer properties and gellation. Also,however, improved properties, for example, improved corrosionresistance, have been achieved with coating compositions comprisingoligomers prepared using such materials and the choice of suitableoligomer (and reactants for preparing same) will depend upon theparticular application intended for the coating composition comprisingthe oligomer. Also preferred are hydantoin epoxy resins and any of awide variety of acyclic or cyclic aliphatic diepoxides such as, forexample, 1,4-butanediol diglycidyl ether an 4-vinylcyclohexene dioxideand the like or a mixture of any of them.

Preferably the diepoxide has a number average molecular weight (M_(n))between about 100 and about 1000, and more preferably between about 100and about 600. Numerous such preferred diepoxides are readilycommercially available, for example, Bisphenol A epichlorohydrin epoxyresin, for example, the Epon (trademark) series, Shell Chemical Company,Houston, Tex., and the DER (trademark) series, Dow Chemical Company,Midland, Mich. Also preferred are cycloaliphatic diepoxy resins, forexample the Eponex (trademark) series, Shell Chemical Company, Houston,Tex., and Resin XB2793 (trademark), Ciba-Geigy Corporation, Ardsley,N.Y. (hydantoin epoxy resin).

The diol can be any of a wide variety of readily commercially availabledihydroxy functional materials of which many are known to the skilled ofthe art. Preferred diols include those of molecular weight about from 60to 500, more preferably about from 60 to 200. Most preferred areterminal diols, that is, diols bearing two terminal hydroxyl groups, forexample, 1,6-hexanediol, since these are generally more reactive. Othersuitable aliphatic diols include primary/secondary andsecondary/secondary carbon hydroxy substituted diols. Diols bearingtertiary hydroxyl groups are least preferred due to their lowerreactivity. Preferred aliphatic diols include, for example, aliphaticdiols of about two to twenty carbons, for example, ethylene glycol,1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 2-ethyl-1,3-hexanedioland the like, or a compatible mixture of any of them. Other suitablealiphatic diols include ether diols, for example those of four to abouttwenty carbons, for example, triethylene glycol and the like. Suitablearomatic diols include those wherein one or both hydroxy groups aresubstituted on a benzene ring. Preferred aromatic diols comprise twohydroxyl groups substituted on the same benzene ring or on differentbenzene rings linked through a covalent bond or through one or morecarbons of a one to seven carbon, preferably three to five carbon,aliphatic moiety. Suitable aromatic diols include, for example,4,4'-isopropylidenediphenol, for example, Bisphenol A (trademark, ShellChemical Company), 4,4'-(1-methyl-propylidene)bisphenol Bisphenol B(trademark, Shell Chemical Company), catechol and the like, or acompatible mixture of any of them. In general, aliphatic diols have beenfound to provide cured coatings of greater flexability and bettercorrosion resistance in comparison to aromatic diols.

The diepoxide is reacted according to methods well known to the skilledof the art, preferably by slow addition to sufficient excess of diolsuch that substantially every epoxide group reacts with an hydroxylgroup of a different diol molecule. The resultant epoxy-diol adductcomprises four hydroxyl groups: the unreacted hydroxyl group of each ofthe two diol molecules which reacted with the diepoxide, and thehydroxyl group formed by each of the two cleaved epoxide rings.Employing a terminal diol, that is, a diol bearing two terminal hydroxylgroups, and terminal diepoxide, the polyol reaction product has twoterminal hydroxyls, each linked through a diol residue to the dihydroxysubstituted diepoxide residue. Whether or not the polyol comprisesepoxy/diol adduct, as just described, it is preferred that two of thethree or more hydroxyls of the polyol be remote from one another andmost preferred that they be terminal hydroxyls as defined above.

The half-blocked diisocyanate with which the above described polyol isreacted to produce oligomers according to the invention preferably has amolecular weight of about from 100 to 500, and can be the reactionproduct of any of a wide variety of organic diisocyanates withapproximately one molar equivalent of a monofunctional blocking agent.Suitable diisocyanates are readily commercially available and includemany known to the skilled of the art such as, for example, phenylenediisocyanate, toluene diisocyanate, isophorone diisocyanate,diisocyanatoalkane wherein the alkyl moiety has preferably about threeto about ten carbons, for example, 1,6-diisocyanatohexane and the like,or a compatible mixture of any of them. If corrosion resistance is ofprimary concern in the cured coating, for example in the case of anautomotive vehicle primer or topcoat, it may be preferred to use analiphatic diisocyanate, for example, isophorone diisocyanate and1,6-hexane diisocyanate. Aromatic diisocyanates provide suitablecoatings, however, and may be preferred in view of their lower cost.

The diisocyanate is half-blocked prior to reaction with the polyol. Thatis, one of the two isocyanate functionalities of the diisocyanatemolecule is reacted with monofunctional blocking agent. The unblockedisocyanate functionality can then react with an hydroxyl group of thepolyol to yield an oligomer according to the present invention bearingthe blocked isocyanate functionality. Suitable half-blockeddiisocyanates can be obtained from the reaction of organic diisocyanatewith sufficient mono-functional blocking agent to block approximatelyone half of the isocyanate functionality. Accordingly, approximately onemolar equivalent of mono-functional blocking agent is reacted with onemolar equivalent of the organic diisocyanate. Suitable techniques wellknown to the skilled of the art can be employed to maximize the yield ofhalf-blocked diisocyanate, such as, for example, adding the blockingagent slowly to the organic diisocyanate under reaction conditions.

Suitable mono-functional blocking agents are well known to the skilledof the art, and readily commercially available. The blocking agent isselected such that the blocked isocyanate group will remain blocked forlong periods of time at normal storage temperatures, but will besubstantially totally "de-blocked" at elevated "cure" temperature. Inaddition, since the blocking agent will be released when the oligomer iscured by baking, it is preferred that the blocking agent have highvolatility near its de-blocking temperature and so will diffuse rapidlythrough the coating composition and evaporate completely therefromduring the baking step. Any blocking agent allowed to remain in thecured coating must be inert to the cured coating and to the substrateand to any other coatings to be used in conjunction with it. It iswithin the skill of those skilled in the art to select a suitableblocking agent to provide a de-blocking temperature meeting therequirements of each particular application of the present invention.Typically, the blocked isocyanate functionality of the oligomers of theinvention will be de-blocked at a temperature within the range of about120° to 200° C. Preferred monofunctional blocking agents are selectedfrom amides, for example caprolactams, phenols, ketoximes, and loweralcohols, for example, alcohols of one to about eight carbons, forexample methanol, ethanol, any propanol, any butanol, any pentanol,including cyclopentanol, and the like, or a mixture of any of them.

The half-blocked diisocyanate is reacted with the previously describedpolyol according to methods well known to the skilled of the art toproduce an oligomer according to the present invention. The free (i.e.,unreacted) isocyanate group of the half-blocked diisocyanate reacts withan hydroxyl group of the polyol forming a urethane linkage. The productoligomer comprises two or more hydroxyls and a single blocked isocyanatefunctionality. During cure, therefore, each oligomer molecule provides asingle isocyanate for chain-extension reaction with an hydroxyl group.It will be recognized by the skilled of the art that where the oligomeris prepared by reaction of an epoxy/diol adduct with a half-blockeddiisocyanate according to the preferred embodiments described above,even where the epoxy/diol adduct comprises two hydroxyl end groups, amixed reaction product could result, since the unblocked isocyanatecould react with any of the one or more non-end group hydroxyl of thepolyol. While not wishing to be bound by theory, however, it ispresently understood that the isocyanate reacts predominately with theleast sterically hindered hydroxyl, that is, with an end group hydroxylof the polyol.

The oligomer comprises latent interreactive bifunctionality suitable forsubstantially linear chain-extension polymerization and furthercomprises hydroxy functionality suitable for crosslinking reactionseparate and distinct from the chain-extension reaction. It is preferredthat the blocked isocyanate and one of the two or more hydroxyls of theoligomer be remote from one another. It is most preferred that they eachbe an end group at opposite ends of the oligomer. Reaction between suchhydroxyl end group and blocked isocyanate end group functional oligomersprovide most efficient chain-extension during cure.

The number average molecular weight of the oligomer according to thepresent invention will affect the volatile organic content of thecoating composition comprising same. Preferably, the number averagemolecular weight is within the range of about 300 to 1100, since thishas been found to provide high-solids coating compositions which can beeasily applied to a substrate by spray or other means at a calculatedvolatile organic content of about 350 g/l (2.9 lb./gal.) or less.

According to the coating composition aspect of the present invention,the above-described novel chain-extendable, crosslinkable urethanemodified polyhydroxy oligomers of the invention are employed in asolvent-based coating composition, preferably together with suitablemulti-functional crosslinking agent. Suitable crosslinking agent is thatwhich will react with the hydroxy functionality of the long chainpolymer product of the chain-extension polymerization reaction of theoligomers during cure. Suitable crosslinking agents will not react,however, with the de-blocked isocyanate group. Accordingly, crosslinkingreaction in the preferred coating composition is a reaction separate anddistinct from the hydroxyl-isocyanate chain-extension reaction. Numeroussuch crosslinking agents are well known to the skilled of the art andinclude, for example, any of a variety of aminoplast crosslinkingagents, for example, partially alkylated melamines (melamineformaldehyde resins modified by alcohols), for example, partiallymethylated melamines and butylated melamines; polyalkyl ethers of thepolymethylol melamines, for example, hexamethoxy methylmelamine; ureaformaldehyde condensate modified by alcohol, for example, butylated urearesin; polymerides of formaldehyde, for example, paraformaldehyde andtrioxane; polymethylol compounds of hexamethylene diurea; adipic aciddimethylol amide and methylol ether thereof;tetramethylolhydrazodicarbonamide; polymethylol compounds ofpolycaprolactam and methylol ethers thereof and the like or mixtures ofany of them. Other suitable crosslinking agents will be apparent to theskilled of the art in view of the present disclosure. Hexamethoxymethylmelamine is preferred since it is readily commercially available and hasbeen found to provide suitable crosslinking activity.

The proper proportion of crosslinking agent to oligomers will dependupon the properties desired in the coating to be produced. Wherehexamethoxymethyl melamine is employed with preferred oligomers of theinvention comprising two crosslinking hydroxy functionalities peroligomer, a weight ratio of crosslinking agent to oligomer of about from1:1 to 1:15, respectively, is generally preferred, and about 1:1 to 1:5is generally more preferred. Too much crosslinking agent will preventproper chain-extention and can produce a coating which is brittle andhumidity sensitive. If too little is used, the coating will not cureproperly.

It will be within the skill of the art to determine the proper volatileorganic content for a given oligomer (or mixture of oligomers) of theinvention, for a given application. In general, preferred solvents arethose having a boiling point between about 60° C. and 200° C., morepreferably between about 110° C. and 170° C. at atmospheric pressure.Preferred solvents include, for example, butanol, methyl amyl ketone andthe like, or a mixture thereof such as a 1:2 mixture of butanol andmethyl amyl ketone, respectively, which is generally preferred.Additional suitable solvents will be apparent to the skilled of the artin view of the present disclosure.

Any solvent allowed to remain in the cured coating should be insert soas to avoid adverse effect upon the cured coating or upon anothercoating used in conjunction with it, during the curing process orthereafter. Preferrably, the finished cured product is completely freeof solvent. Preferred solvents, in addition, have relatively lowvolatility at temperatures appreciably below their boiling points suchthat solvent evaporation is low during storage and/or application of thecoating composition to the substrate.

Sufficient solvent is used to reduce the viscosity of the coatingcomposition to a level suitable for application to the substrate in thedesired manner. While conventional epoxy ester-type automotivespray-applied primer coating compositions are known to require avolatile organic content of about 540 g/l, comparable coatingcompositions comprising instead the novel, low molecular weightoligomers of the present invention require as little as 350 g/l or lessVOC to provide a spray viscosity of 25-35 sec, #4 Ford Cup. Of course,the coating compositions of the invention need not be formulated as a"high solids" composition, but rather can have a higher VOC to provide alower viscosity. It is generally preferred that sufficient solvent beused to provide a viscosity of about 15 to 35 seconds, No. 4 Ford Cup at27° C. (80° F.).

Also preferably included in the coating composition of the invention isany of a variety of acid catalysts known to the skilled of the art tocatalyse the aminoplast crosslinking reaction, for example,p-toluenesulfonic acid, phosphoric acid, phenyl acid phosphate, butylphosphate, butyl maleate, and the like or a compatible mixture of any ofthem. In addition, any of a variety of catalysts for the isocyanatede-blocking reaction can also be included in the coating composition,for example, dibutyl tin dilaurate. In addition, flow control agent(s),for example, polybutyl acrylate; wetting agent(s), for example,silicone; pigments; pigment dispersents; corrosion inhibitors, forexample, chromate pigments, numerous of all of which are known to theskilled of the art, may be employed in the coating compositions of theinvention.

It should be recognized that the coating compositions can comprisemonohydroxy blocked isocyanate functional compounds in addition to thepolyhydroxy functional oligomers of the invention. A molecule of such amonohydroxy compound, its hydroxyl group having reacted with ade-blocked isocyanate group to provide chain-extension, would contributeno crosslinking functionality to the high molecular weightchain-extended polymerization product. By simple adjustment of theproportion of monohydroxy compound to polyhydroxy oligomer in thecoating composition, the crosslink density in the cured coating, andtherefore the degree of flexibility of the cured coating can beaccurately controlled. Thus, for example, monohydroxy compounds can beemployed which are the reaction product of a half-blocked diisocyanateas described above and a diol (rather than a polyol having three or morehydroxy functionalities) whereby a monohydroxy blocked urethanefunctional oligomer is formed (rather than an oligomer according to thepresent invention). Such monohydroxy oligomer, having undergonechain-extension reaction with the novel oligomer of the invention, wouldprovide no hydroxy crosslinking functionality. Monohydroxy blockedisocyanate functional compounds for use in high solids coatingcompositions are preferably of number average molecular weight of about200 to 1000, more preferably about 200 to 500.

According to another aspect of the invention, a coating on a substrateis provided, which coating comprises the chain-extended, crosslinkedpolymer product following cure of a coating comprising the coatingcomposition of the invention. The coating composition can be a lowsolids composition, that is, it can have a high VOC, but generally ahigh solids composition, that is, one having a low VOC is preferred forthe reasons given above. It can be applied by any conventional method,including brushing, dipping, flow coating, spraying, etc. Spraying willgenerally be preferred, for example, for applying the composition as anautomotive primer or topcoat. In such spraying applications, the noveloligomers of the invention are especially advantageous for formulatingcoating compositions of the invention which are high solidscompositions. In this regard, coating compositions of the inventionemploying preferred oligomers described above are suitable to be appliedto a substrate by spraying even though formulated at volatile organiccontent levels as low as about 330 to 360 g/l (2.7 to 3.0 lb/gal).

Curing the coating composition requires baking for sufficient time atsufficiently elevated temperature to de-block the blocked isocyanatefunctionality of the oligomers. The time and temperature required tocure the coating are interrelated and depend upon the particularoligomer(s), crosslinking agent(s), solvent and other materials, if any,and the amount of each comprising the coating composition. Employing avolatile organic content of about 350 g/l and selecting preferredcomponents as described above, the bake time and temperature istypically about 20 to 30 minutes and about 180° C. The temperaturerequired for cure can be reduced to about 150° C. for 20 to 30 minutesby addition of suitable catalyst such as any of those known to theskilled of the art, for example, dibutyl tin dilaurate and even lower byuse of suitable blocking groups etc.

High solids coating compositions according to the present invention,comprising the novel chain-extendable, crosslinkable oligomers of theinvention, especially the preferred urethane modified epoxy/diol adductsdescribed above and, preferably, an aminoplast crosslinking agent, forexample, hexamethoxymethyl melamine, have been found to afford curedcoatings with corrosion resistance comparable to conventional epoxyester based, low solids sprayable coating compositions. The significantreduction in volatile organic content presents, therefore, a highlyadvantageous advance in the art.

As it is presently understood, the single blocked isocyanatefunctionality of the oligomer, which is de-blocked at cure temperatures,provides substantially linear chain-extension, in situ, on the surfaceof the substrate during cure by reaction with an hydroxyl groups of theoligomers. The additional one or more hydroxyl groups of each oligomerof the invention are available for crosslinking reaction with suitablecrosslinker. While not wishing to be bound by theory, it is presentlyunderstood that upon curing a coating composition according to thepresent invention, the blocked isocyanate group of the novel oligomersof the invention is de-blocked and reacts more readily with the leaststerically hindered of the two or more hydroxyl groups of anotheroligomer. If the oligomer comprises a terminal hydroxy group, as inpreferred embodiments described above, then the de-blocked isocyanategroup would react most readily with that terminal hydroxyl group and notwith any of the one or more additional, non-terminal hydroxyl group(s)of the oligomers or of an extended chain polymer already formed by thecuring process. Such additional non-terminal hydroxy functionalityremains available for crosslinking reaction. If, for example, the polyolemployed to prepare the oligomer is an epoxy/diol adduct reactionproduct of a terminal diol, for example, 1,6-hexanediol, with a terminaldiepoxide, for example, an hydantoin epoxy resin, then the oligomer willhave a terminal hydroxy, two non-terminal hydroxy (formed by cleavage ofthe epoxide rings) and a terminal blocked isocyanate. During cure,according to present understanding, the isocyanate functionally will bede-blocked and will then react predominantly with the terminal hydroxylgroup. The result is substantially linear chain-extension polymerizationof the oligomers, in situ, on the surface of the substrate, to form longchain, high molecular weight polymers with pendant hydroxyl groupssuitable for crosslinking reaction. Accordingly, the polymer networksobtained during cure of the coating compositions comprising theoligomers of the present invention are believed to be similar instructure to those obtained using conventional low solids solvent basedcoating compositions.

Even where the isocyanate group does not react with a terminal hydroxylgroup, however, the one isocyanate group of each oligomer can react onlywith one other oligomer; since the isocyanate group is not reactive withthe crosslinking agent, the result is substantially linearchain-extension polymerization.

In addition, newtwork crosslink density can be controlled, and thereforethe flexibility of the cured coating can to a large extent be controlledby proper selection of oligomer(s). Crosslink density increases andflexibility decreases as the hydroxy functionality is increased and/oras the molecular weight of the oligomer is reduced. Thus, it will beapparent, to the skilled of the art that if the oligomer is prepared byreaction of a polyol with a half-blocked diisocyanate, according topreferred embodiments described above, the selection of the polyol andhalf-blocked diisocyanate reactants provides substantial control of thecrosslink density in the cured coating. The greater the hydroxyfunctionality of the polyol and the lower the molecular weight of thereactants, the greater will be the degree of crosslinking. Thus, forexample, where the polyol is the reaction product of a diepoxide and adiol, there will be a higher crosslink density in the cured coating ifthe diol is 1,3-propanediol than if it is 1,6-hexanediol.

In addition, it will be recognized by the skilled of the art in view ofthe present disclosure that longer chain oligomers, that is, highermolecular weight oligomers will, in general, provide a more viscouscoating at a given VOC. Higher molecular weight oligomers of theinvention are for that reason less preferred where a high solids coatingcomposition is desired.

Cured coatings according to the invention have been found to provideexcellent corrosion resistance when applied over a metallic substratesuch as, for example, when applied as an automotive vehicle primer coatover bare sheet steel. While not wishing to bound by theory, theexceptional corrosion inhibitors provided by preferred embodimentsdescribed above stem, in part, from the absence of ester linkages. Esterlinkages are known to be attacked by hydroxide, a product of the metalcorrosion process.

EXAMPLE I Preparation of Epoxy/Diol Adduct

This example illustrates the preparation of an epoxy/diol adduct from aheterocyclic epoxy and a branched acyclic aliphatic diol. Hydantoinepoxy resin XB2793 (trademark, Ciba-Geigy Corporation), 138. g,1,3-hexanediol, 146. g, and N,N-dimethylethanolamine, 0.5 g, werecombined in methyl amyl ketone, 71. g, and refluxed approximately 40hours until the epoxy infrared absorption disappeared. The low viscosityresin product was cooled to room temperature and stored.

EXAMPLE II Preparation of Epoxy/Diol Adduct

The example illustrates the preparation of an epoxy/diol adduct from aheterocyclic epoxy and an aromatic diol. Hydantoin epoxy resin XB2793(trademark, Ciba-Geigy Corporation), 69. g, and Bisphenol A (trademark,Shell Chemical Company), 114. g, were combined in methyl amyl ketone,45.8 g, and refluxed approximately 4 hours until the epoxy infraredabsorption disappears. The resin product was cooled to room temperatureand stored.

EXAMPLE III Preparation of Half-blocked Diisocyanate

This example illustrates the preparation of an alcohol half-blockedaliphatic diisocyanate. Butyl alcohol, 74. g was added dropwise to amixture of isophorone diisocyanate, 222. g, and dibutyl tin dilaurate,0.5 g, and methyl amyl ketone, 74. g. After addition of the alcohol, themixture was heated to between 60°-80° for 2 hours. (Higher temperatureswere avoided to avoid undesirable side reactions.) The half-blockeddiisocyanate product was characterized by infrared spectroscopy showingthe absence of OH absorption at 3300 cm⁻¹, a reduction in N═C═Oabsorption at 2250 cm⁻¹ and the presence of urethane N--H and carbonylabsorptions at 3250 cm⁻¹ and 1730 cm⁻¹, respectively.

EXAMPLE IV Preparation of Urethane Modified Epoxy/Diol Resin

A novel oligomer according to a preferred embodiment of the invention,an isophorone diisocyanate modified-hydantoin epoxy/Bisphenol A resin(trademark, Shell Chemical Company), was prepared by combining 75. g ofthe half-blocked diisocyanate product of Example III with 228.8 g of theepoxy/diol adduct product of Example II. The mixture was stirred withheating at about 100° C. for about 3 to 4 hours, until substantially allthe isocyanate had reacted, as determined by infrared spectroscopy. Theurethane modified epoxy/diol resin product was cooled to roomtemperature and stored.

EXAMPLE V Preparation of Urethane Modified Epoxy/Diol Resin

A novel oligomer according to a preferred embodiment of the invention,an isophorone diisocyanate modified-hydantoinepoxy/2-ethyl-1,3-hexanediol resin, was prepared by combining 197. g ofthe half-blocked diisocyanate product of Example III with 284. g of theepoxy/diol adduct product of Example I. The mixture was stirred withheating to about 100° C. for about 3 to 4 hours, until substantially allthe isocyanate had reacted, as determined by infrared spectroscopy. Theurethane modified epoxy/diol resin product was cooled to roomtemperature and stored.

EXAMPLE VI

A novel oligomer according to a preferred embodiment of the invention,was prepared by a three step procedure.

A. Preparation of Epoxy/Diol Adduct

An aromatic epoxy/branched chain aliphatic diol adduct was prepared byrefluxing a mixture of Epon 828 (trademark, Shell Chemical Company),190. g, 2-ethyl-1,3-hexanediol, 146. g, methyl amyl ketone, 84. g, andN,N-dmethylethanol amine, 0.5 g, for about 4 to 8 hours until theinfrared epoxide absorption disappears.

B. Preparation of Half-blocked diisocyanate

Butanol half-blocked isophorone diisocyanate was prepared according tothe procedure of Example III.

C. Preparation of Urethane Modified Epoxy/Diol Resin

The novel oligomer of the invention was prepared by reacting at 100° C.the Epon 828/2-ethyl-1,3-hexanediol adduct product of step A, 163.2 g,with butanol half-blocked diisocyanate product of step B, 92.5 g.Reaction was allowed to proceed until no isocyanate infrared absorptionis detected. The urethane modified epoxy/diol resin product was cooledto room temperature and stored.

EXAMPLE VII Preparation of Half-blocked Diisocyanate

This example illustrates the preparation of an alcohol half-blockedaromatic diisocyanate. Butanol, 14.8 g, was added dropwise, withstirring, to toluene diisocyanate, 35. g, in 12.5 g of methyl amylketone, at a rate controlled to maintain the reaction temperature atabout 60° C. After addition of the butanol, the reaction mixture wasstirred an additional hour at room temperature. The half-blockeddiisocyanate product was verified by infrared spectroscopy as in ExampleIII. The product was stored for later use.

EXAMPLE VIII Preparation of Urethane Modified Epoxy/Diol Resin

A novel oligomer according to a preferred embodiment of the invention, atoluene diisocyanate modified-hydantoin epoxy/2-ethyl-1,3-hexanediolresin, was prepared by combining 133. g of the hydantoin epoxy/dioladduct product of Example I with 62.3 g of the alcohol half-blockedaromatic diisocyanate product of Example VII. The reaction mixture washeated at about 80°-90° C. for about 2 hours, until substantially allthe isocyanate had reacted, as determined by infrared spectroscopy. Theurethane modified epoxy/diol resin product was cooled to roomtemperature and stored.

EXAMPLE IX Preparation of Urethane Modified Epoxy/Diol Resin

A novel oligomer according to a preferred embodiment of the invention, atoluene disocyanate modified-Epon 828/2-ethyl-1,3-hexane diol resin, wasprepared by combining 213. g of the Epon 828/2-ethyl-1,3-hexanedioladduct product of Example VI part A with 91.4 g of 2-ethylhexanolhalf-blocked toluene diisocyanate in 15. g of methyl amyl ketone. (Thehalf-blocked diisocyanate was prepared according to the procedure ofExample VII, employing 2-ethylhexanol in place of butanol.) The reactionmixture was heated at 80°-90° C. for about 2 hours, until substantiallyall the isocyanate had reacted, as determined by infrared spectroscopy.The urethane modified epoxy/diol resin product was cooled to roomtemperature and stored.

EXAMPLE X Preparation of Epoxy/Diol Adduct

This example illustrates the preparation of an epoxy/diol adduct from anaromatic epoxy and a straight chain aliphatic diol. Epon 828 (trademark,Shell Chemical Company), 190. g, 1,5-pentanediol, 78. g, anddimethylethanolamine, 0.68 g, were combined in methyl amyl ketone, 67.g. The reaction mixture was heated at 100°-130° C. for 16 hours. Theproduct, under infrared spectroscopy, revealed no absorption for epoxy.The product was stored for later use.

EXAMPLE XI Preparation of Urethane Modified Epoxy/Diol Resin

A novel oligomer according to a preferred embodiment of the invention, atoluene diisocyanate modified-Epon 828/1,5-pentanediol resin, wasprepared by combining 367.5 g of the Epon 828/1,5-pentanediol adductproduct of Example X with 170. g of 2-ethyl-1,3-hexanediol half-blockedtoluene diisocyanate in 34.5 g methyl amyl ketone. (The half-blockeddiisocyanate was prepared according to the procedure of Example VII,employing an equivalent amount of 2-ethylhexanol in place of butanol.)The reaction mixture was heated at 80°-90° C. for about 2 hours, untilsubstantially all the isocyanate had reacted, as determined by infraredspectroscopy. The urethane modified epoxy/diol resin product was cooledto room temperature and stored.

EXAMPLE XII Preparation of Epoxy/Diol Adduct

This example illustrates the preparation of an epoxy/diol adduct from analiphatic epoxy and a branched chain aliphatic diol. Aliphatic epoxyresin, Eponex 151 (trademark, Shell Chemical Company), 234. g,2-ethyl-1,3-hexanediol, 146. g, and N,N-dimethylethanolamine, 1. g, werecombined and heated at 120°-140° C. for about 20 hours. The product,under infrared spectroscopy, showed no absorption for epoxy. The productwas stored at room temperature.

EXAMPLE XIII Preparation of Epoxy/Diol Adduct

This example illustrates the preparation of an aliphatic epoxy/aliphaticdiol adduct. Aliphatic epoxy resin, Eponex 151 (trademark, ShellChemical Company), 234. g, 1,5-pentanediol, 104. g, andN,N-dimethylethanolamine were heated at 120°-140° C. for about 20 hours.The product, under infrared spectroscopy, showed no absorption forepoxy. The product was stored at room temperature.

EXAMPLE XIV Preparation of Novel Oligomer

Eponex 151/2-ethyl-1,3-hexanediol adduct, prepared as in Example XII,190. g, was combined with 2-ethylhexanol half-blocked toluenediisocyanate, 85. g, in 29. g of methyl amyl ketone. (The half-blockeddiisocyanate was prepared according to the procedure of Example VII,employing 2-ethylhexanol in place of butanol.) The reaction mixture washeated at 80°-100° C. for 3 hours, until substantially all theisocyanate had reacted, as determined by infrared spectroscopy. Theurethane modified epoxy/diol resin product was cooled to roomtemperature and stored.

EXAMPLE XV Preparation of Novel Oligomer

Eponex 151/2-ethyl-1,3-hexane diol adduct, prepared as in Example XII,190. g, was combined with butanol half-blocked isophorone diisocyanateprepared as in Example III, 92.5 g, in 29. g methyl amyl ketone. Thereaction mixture was heated at 80°-100° C. for 3 hours, untilsubstantially all the isocyanate had reacted, as determined by infraredspectroscopy. The oligomer product was cooled to room temperature andstored.

EXAMPLE XVI Preparation of Novel Oligomer

Eponex 151/2-ethyl-1,3-hexanediol adduct, prepared as in Example XIII,169. g, was combined with 2-ethylhexanol half-blocked toluenediisocyanate, 85. g, in 70. g of methyl amyl ketone. The reactionmixture was heated at 80°-100° C. for 3 hours, until substantially allthe isocyanate had reacted, as determined by infrared spectroscopy. Theurethane modified epoxy diol resin product was cooled to roomtemperature and stored.

EXAMPLE XVII Preparation of Novel Oligomer

Eponex 151/1,5-pentanediol adduct, prepared as in Example XIII, 169. g,was combined with butanol half-blocked isophorone diisocyanate, preparedas in Example III, 92.5 g, in 29. g methyl amyl ketone. The reactionmixture was heated at 80°-100° C. for 3 hours, until substantially allthe isocyanate had reacted, as determined by infrared spectroscopy. Theoligomer product was cooled to room temperature and stored.

EXAMPLE XVIII Coating Composition

A coating composition suitable for use as an automotive primer coatingcomposition was prepared according to the following formulation usingconventional techniques well known to the skilled of the art.

Primer Formulation No. 1

    ______________________________________                                        Part A: Pigment Package                                                       Grams          Pigment                                                        ______________________________________                                        4.3            Silica                                                         48.4           Barytes                                                        0.6            Carbon black (Neotex 130,                                                     Cities Service, Columbian                                                     Chemical Division, Akron,                                                     Ohio)                                                          6.5            Titanium dioxide                                               ______________________________________                                    

    ______________________________________                                        Part B: Binder Package                                                        Grams          Organic Material                                               ______________________________________                                        58             Toluene diisocyante modified                                                  Epon 828/1,5-pentanediol                                                      Resin (prepared as in                                                         Example XI)                                                    15             Cymel 301 (trademark,                                                         Ciba-Geigy Corp., Ardsley,                                                    New York)                                                                     (hexamethoxymethyl melamine)                                    5             20% Paratoluene sulfonic                                                      acid solution in methyl ethyl                                                 ketone                                                         30             1:2 Butanol: Methyl amyl                                                      ketone                                                         ______________________________________                                    

A mill base was prepared by dispersing the binder package with thepigment package. Milling was continued until the resulting primer had aHegman Gage reading of about 6.5 to 7. The primer was filtered andapplied to steel test panels by conventional air atomized spray andcured at 150°-180° C. for 20 minutes.

EXAMPLE XIX Coating Composition

A coating composition suitable for use as an automotive primer coatingcomposition was prepared in the manner described in Example XVIIIaccording to the following formulation, which comprises a corrosioninhibiting pigment, strontium chromate.

Primer Formulation No. 2

    ______________________________________                                        Part A: Pigment Package                                                       Grams          Pigment                                                        ______________________________________                                        39.4           Barytes                                                        0.6            Carbon black (Neotex 130,                                                     Cities Service, Columbian                                                     Chemical Division, Akron,                                                     Ohio)                                                          6.5            Titanium dioxide                                               9.0            Strontium chromate                                             4.3            Silica                                                         ______________________________________                                    

    ______________________________________                                        Part B: Binder Package                                                        Grams          Organic Material                                               ______________________________________                                        58             Toluene diisocyanate modified                                                 Epon 828/2-ethyl-1,3-                                                         hexanediol resin (prepared as                                                 in Example IX)                                                 15             Cymel 301 (trademark,                                                         Ciba-Geigy Corp., Ardsley,                                                    New York) (hexamethoxymethyl                                                  melamine)                                                       5             20% Paratoluene sulfonic                                                      acid solution in methyl ethyl                                                 ketone                                                         30             2:1 Methyl amyl                                                               ketone: butanol                                                ______________________________________                                    

A mill base was prepared by dispersing the binder package with thepigment package. Milling was continued until the resulting primer had aHegman Gage reading of about 6.5 to 7. The primer was filtered andapplied to steel test panels by conventional air atomized spray andcured at 150°-180° C. for 20 minutes.

EXAMPLE XX-XXIX Corrosion Resistance

Ten coating compositions were prepared according to the followingformulations.

    __________________________________________________________________________              EXAMPLE (weight in grams)                                           Component XX  XXI                                                                              XXII                                                                              XXIII                                                                             XXIV                                                                              XXV                                                                              XXVI                                                                              XXVII                                                                             XXVIII                                                                             XXIX                             __________________________________________________________________________    Resin     100.                                                                              51.5                                                                             60. 51.5                                                                              58.    60. 60. 60.  60.                              Cymel 301 24. 13.                                                                              15. 13. 15.    15. 15. 15.  15.                              20% Para-toluene                                                                        2.5 2.7                                                                              2.  2.8 5.0    2.  2.  2.   2.                               sulfonic acid in                                                              methyl ethyl ketone                                                           Silica    5.3 3.9                                                                              3.8 --  4.3    3.8 3.8 3.8  3.8                              Barytes   59.8                                                                              44.3                                                                             42.7                                                                              42.8                                                                              39.4   42.7                                                                              42.7                                                                              42.7 42.7                             Carbon black                                                                            .77 .6 .5  .6  .6     .5  .5  .5   .5                               Titanium Dioxide                                                                        8.  5.9                                                                              5.8 5.7 6.5    5.8 5.8 5.8  5.8                              Butanol:methyl                                                                          50. 30.                                                                              30. 30. 30.    30. 30. 30.  30.                              amyl ketone (1:2)                                                             Dibutyl tin                                                                             --  -- .24 --  --     .4  .4  .3   .3                               dilaurate                                                                     __________________________________________________________________________

The resin in each coating composition consisted of a novel oligomeraccording to the present invention prepared according to the foregoingexamples. The particular resin in each coating composition is listedbelow. The Roman numeral in parenthesis following the resin indicatesthe Example according to which it was prepared. (The figure given inTable 1 for VOC is calculated from the formulation.)

                                      TABLE 1                                     __________________________________________________________________________    Coating                             24 Hour                                                                             Cathodic                            Composition                                                                          Resin                  VOC (g/l)                                                                           Salt Spray.sup.a                                                                    Polarization.sup.a                  __________________________________________________________________________    XX     Isophorone diisocyanate modified-hydantoin                                                           418   TF.sup.b                                                                            TF.sup.b                                   epoxy/bis-phenol A (Example IV)                                        XXI    Isophorone diisocyanate modified-hydantoin                                                           340   3     2-3                                        epoxy/2-ethyl-1,3-hexanediol (Example V)                               XXII   Isophorone diisocyanate modified-Epon 828/                                                           346   1     NA.sup.c                                   2-ethyl-1,3-hexanediol (Example VI)                                    XXIII  Toluene diisocyanate modified-hydantoin epoxy/                                                       415   3-4   4-5                                        2-ethyl-1,3-hexanediol (Example VIII)                                  XXIV   Toluene diisocyanate modified-Epon 828/                                                              362   4-6   7-8                                        2-ethyl-1,3-hexanediol (Example IX)                                    XXV    Toluene diisocyanate modified-Epon 828/                                                              362   5-6   3-4                                        1,5-pentanediol (Example XI)                                           XXVI   Toluene diisocyanate modified-Eponex 151/                                                            327   1-2   1                                          2-ethyl-1,3-hexanediol (Example XIV)                                   XXVII  Isophorone diisocyanate modified-Eponex 151/                                                         372   3-4   .sup. 2-3.sup.d                            2-ethyl-1,3-hexanediol (Example XV)                                    XXVIII Toluene diisocyanate modified-Eponex 151/                                                            327   2-4   .sup. 1-2.sup.d                            2-ethyl-1,3-hexanediol (Example XVI)                                   XXIX   Isophorone diisocyanate modified-Eponex 151/                                                         327   1-2   2-3                                        1,5-pentanediol (Example XVII)                                         __________________________________________________________________________     .sup.a Adhesion loss from scribe in millimeters                               .sup.b TF = Total failure                                                     .sup.c NA = Not available                                                     .sup.d Non-scribe line associated adhesion loss observed                 

The following examples illustrate the adverse effect on corrosionresistance of introducing non-chain-extendable resin (epoxy/diol adduct)into the primer compositions of the invention.

EXAMPLE XXX

The novel oligomer toluene diisocyanate-modified Epon828/1,5-pentanediol, prepared in accordance with Example XI, was usedwith Epon 828/1,5-pentanediol prepared in accordance with Example X. Thecorrosion test results are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Percent      Percent      24 Hour                                             Oligomer     Epoxy/Diol Resin                                                                           Salt Spray.sup.a                                    ______________________________________                                        100           0           5-6                                                 75           25           5-6                                                 50           50           .sup. 5-6.sup.b                                     25           75           .sup. TF.sup.c                                       0           100          TF                                                  ______________________________________                                         .sup.a Adhesion loss from scribe in millimeters                               .sup.b Non-scribe line adhesion loss observed                                 .sup.c TF = Total failure                                                

EXAMPLE XXXI

The novel oligomer toluene diisocyanate-modified Epon828/2-ethyl-1,3-hexanediol, prepared in accordance with Example IX wasused with Epon 828/2-ethyl-1,3-hexanediol prepared in accordance withExample VI Part A. The corrosion test results are presented in Table 3.

                  TABLE 3                                                         ______________________________________                                        Percent      Percent      24 Hour                                             Oligomer     Epoxy/Diol Resin                                                                           Salt Spray.sup.a                                    ______________________________________                                        100           0           4-6.sup.                                            75           25           6-8.sup.b                                           50           50           6-8.sup.b                                           25           75           TF.sup.c                                             0           100          TF                                                  ______________________________________                                         .sup.a Adhesion loss from scribe in millimeters                               .sup.b Non-scribe line adhesion loss observed                                 .sup.c TF = Total failure                                                

EXAMPLE XXXII

The novel oligomer toluene diisocyanate-modified hydantoinepoxy/2-ethyl-1,3-hexanediol, prepared in accordance with Example VIII,was used with hydantoin epoxy/2-ethyl-1,3-hexanediol, prepared inaccordance with Example I. The corrosion test results are presented inTable 4.

                  TABLE 4                                                         ______________________________________                                        Percent      Percent      24 Hour                                             Oligomer     Epoxy/Diol Resin                                                                           Salt Spray.sup.a                                    ______________________________________                                        100           0           4-5.sup.b                                           75           25           6-8.sup.b                                           50           50           TF                                                  25           75           TF                                                   0           100          TF                                                  ______________________________________                                         .sup.a Adhesion loss from scribe in millimeters                               .sup.b Non-scribe line adhesion loss observed                                 .sup.c TF = Total failure                                                

Particular embodiments of the present invention described above areillustrative only and do not limit the scope of the invention. It wil beapparent to the skilled of the art in view of the foregoing disclosurethat modifications and substitutions can be made without departing fromthe scope of the invention.

We claim:
 1. A solvent-based thermosetting resin composition comprising:a crosslinkable urethane modified polyhydroxy oligomer which is chain-extendable at between about 100° C. and about 190° C., said oligomer having a molecular weight between about 200 and about 1500 and bearing three or more hydroxyl groups and one blocked isocyanate group, said isocyanate group being blocked with a monofunctional blocking agent which de-blocks at between about 100° C. and about 190° C.; and crosslinking agent reactive with the hydroxyl functionality of said oligomer and substantially unreactive with isocyanate functionality.
 2. A solvent-based thermosetting resin composition according to claim 1, wherein said oligomer bears one terminal hydroxyl group and two non-terminal hydroxyl groups, said blocked isocyanate functionality being a terminal functionality.
 3. A solvent-based composition comprising:A. chain-extendable, crosslinkable urethane modified polyhydroxy oligomer containing no ester linkages and bearing three or more hydroxyl groups and a single blocked isocyanage group, said oligomer comprising the reaction product of a polyol with a half-blocked diisocyanate, wherein said polyol bears four or more hydroxyl groups, and said half-blocked diisocyanage comprises the reaction product of an organic diisocyanate with monofunctional blocking agent; B. crosslinking agent reactive with hydroxyl functionality and substantially unreactive with isocyanate functionality; and C. organic solvent.
 4. The resin compositon of claim 3 wherein said monofunctional blocking agent of said reaction product has an unblocking temperature of between about 100° and about 190° C.
 5. The resin composition of claim 3 wherein said oligomer has a molecular weight between about 200 and about
 1500. 6. The resin composition of claim 3 wherein said polyol bears from 4 to 10 hydroxyl groups.
 7. The composition of claim 3 wherein said half-blocked diisocyanate has a molecular weight of about from 100 to 600 and comprises the reaction product of an organic diisocyanate with approximately one molar equivalent of a monofunctional blocking agent selected from the group consisting of alcohol, amide, phenol and a mixture of any of them.
 8. The composition of claim 7 wherein said monofunctional blocking agent is butanol.
 9. The composition of claim 3 wherein said reaction product is soluble in said solvent to the extent that a solution of about 350 g/l or less VOC has viscosity of less than about 35 sec., #4 Ford Cup at about 27° C.
 10. The composition of claim 9 wherein said solvent is butanol.
 11. The composition of claim 3 wherein said crosslinking agent consists of polyalkoxyalkylmelamine, wherein said alkoxy and said alkyl moiety each comprises from about 1 to 3 carbons.
 12. The composition of claim 11 wherein said crosslinking agent consists of hexamethoxymethylmelamine.
 13. The composition of claim 3 wherein said crosslinking agent and said oligomer are present in a weight ratio of about 1:1 to 1:15, respectively.
 14. The composition of claim 3 further comprising a catalyst for said crosslinking agent.
 15. The composition of claim 14 wherein said catalyst is selected from the group consisting of p-toluenesulfonic acid, phosphoric acid, phenyl acid phosphate, butyl phosphate, butyl maleate and a mixture of any of them.
 16. The composition of claim 3 further comprising monohydroxy blocked isocyanate functional compound of molecular weight about from 200 to
 1000. 17. The composition of claim 16 wherein said monohydroxy blocked isocyanate functional compound is the reaction product of a half-blocked diisocyanate and a diol. 